In fuel cells, hydrogen or methanol is electro-chemically oxidized to convert the chemical energy of such a fuel directly to electric energy and this electric energy is recovered. The fuel cells are known as a clean source of supply of electric energy. In particular, solid polymer electrolyte fuel cells are useful as a substitute power source for automobiles, a co-generation system for domestic use, and portable electric generators because they work at a lower temperature as compared with other fuel cells.
Such solid polymer electrolyte fuel cells comprise a solid polymer electrolyte membrane and a pair of gas-diffusion electrodes joined to both sides, respectively, of the membrane. In detail, the solid polymer electrolyte fuel cells have a structure in which an anode catalyst layer is formed on one side of the solid polymer electrolyte membrane and a cathode catalyst layer on the other side, and a pair of electrode supports are provided outside the anode and cathode catalyst layers, respectively, so as to be adjacent to them. The anode and cathode catalyst layers have been those obtained by making a mixture of carbon black powder supporting an electrocatalyst, a proton-conductive polymer and a water-repellent polymer into a sheet, and are joined to the solid polymer electrolyte membrane by hot pressing.
A fuel (e.g. hydrogen) is supplied to the gas-diffusion electrode (as anode) side and an oxidizing agent (e.g. oxygen or air) to the other gas-diffusion electrode (as cathode) side, and the electrodes are connected to each other by an external circuit. The resulting assembly works as a fuel cell. That is, protons are produced in the anode by the oxidation of the fuel and pass through the solid polymer electrolyte to migrate to the cathode side. On the other hand, electrons arrive at the cathode through the external circuit. In the cathode, water is produced from such protons and electrons and oxygen in the oxidizing agent, whereupon electric energy is recovered.
In this case, what is important is that the transfer and conduction of protons and the gas on the surfaces of catalyst particles supported by a supporting substance wholly in the directions of thickness and plane of each catalyst layer are sufficiently achieved, so that electric energy is conducted by the electrode substrates with high efficiency. For this purpose, JP-A-5-36418 discloses a method in which electrodes are obtained by mixing a solid polymer electrolyte, a catalyst, carbon powder and a fluororesin and making the mixture into a film. JP-A-10-302805 has proposed the diameter of colloidal particles of a solid polymer electrolyte which is suitable for forming a layer of the solid polymer electrolyte in a proper thickness on the surface of a catalyst-supporting substance. In addition, JP-A-10-284087 has proposed obtaining the following effects by incorporating at least two proton-conductive polymers different in equivalent weight (EW) into catalyst layers: a polymer with a low EW allows the cell reaction to proceed smoothly and a polymer with a high EW permits rapid discharge of produced water from the catalyst layer to maintain the supply of gases to the catalyst.
However, the advancement of a technique for atomization of a catalyst and supporting-carbon is so remarkable that the ultra-atomization of platinum catalyst to a diameter of 20 to 30 Å has been successful and that the atomization of supporting-carbon to a diameter of 150 to 1000 Å has been realized. It is important to cover the ultra-atomized catalyst and the supporting-carbon with a solid polymer electrolyte as uniformly as possible to improve the utilization factor of the catalyst as much as possible and optimize the transfer and conduction of protons and gases on the surfaces of catalyst particles. It is also important to maintain the transfer of electrons between catalyst particles and a supporting substance, that among particles of a supporting substance and that between the supporting substance and an electrode support. However, although the supporting substance can be covered or bound to a certain extent with a solid polymer electrolyte in a colloidal state, it is difficult to sufficiently cover ultra-fine electrocatalyst particles supported on the supporting substance with such a polymer electrolyte. When a fluororesin is used for binding the supporting substance, the proton conductivity is unavoidably deteriorated. Also when the catalyst layers contain at least two proton-conductive polymers different in equivalent weight (EW), the molecular weights of the proton-conductive polymers have not yet been optimized.
With the ultra-atomization of catalyst particles, the extension of the lifetime of the catalyst becomes a problem. In the course of use of the catalyst, catalyst particles are aggregated to become large, so that their surface area is decreased, resulting in the deterioration of the catalyst. In order to avoid this deterioration, catalyst particles are covered with a solid polymer electrolyte as uniformly as possible as in the case of the electrode for fuel cell of the present invention, whereby the extension of the lifetime can be expected.